Imshow in Python

How to display image data in Python with Plotly.

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Plotly is a free and open-source graphing library for Python. We recommend you read our Getting Started guide for the latest installation or upgrade instructions, then move on to our Plotly Fundamentals tutorials or dive straight in to some Basic Charts tutorials.

This tutorial shows how to display and explore image data. If you would like instead a logo or static image, use go.layout.Image as explained here.

Displaying RBG image data with px.imshow¶

px.imshow displays multichannel (RGB) or single-channel ("grayscale") image data.

In [1]:

import plotly.express as px
import numpy as np
img_rgb = np.array([[[255, 0, 0], [0, 255, 0], [0, 0, 255]],
                    [[0, 255, 0], [0, 0, 255], [255, 0, 0]]
                   ], dtype=np.uint8)
fig = px.imshow(img_rgb)
fig.show()

Read image arrays from image files¶

In order to create a numerical array to be passed to px.imshow, you can use a third-party library like PIL, scikit-image or opencv. We show below how to open an image from a file with skimage.io.imread, and alternatively how to load a demo image from skimage.data.

In [2]:

import plotly.express as px
from skimage import io
img = io.imread('https://upload.wikimedia.org/wikipedia/commons/thumb/0/00/Crab_Nebula.jpg/240px-Crab_Nebula.jpg')
fig = px.imshow(img)
fig.show()

In [3]:

import plotly.express as px
from skimage import data
img = data.astronaut()
fig = px.imshow(img)
fig.show()

Display single-channel 2D image as grayscale¶

For a 2D image, px.imshow uses a colorscale to map scalar data to colors. The default colorscale is the one of the active template (see the tutorial on templates).

In [4]:

import plotly.express as px
import numpy as np
img = np.arange(15**2).reshape((15, 15))
fig = px.imshow(img)
fig.show()

Choose the colorscale to display a single-channel image¶

In [5]:

import plotly.express as px
import numpy as np
img = np.arange(100).reshape((10, 10))
fig = px.imshow(img, color_continuous_scale='gray')
fig.show()

Hiding the colorbar when displaying a single-channel image¶

See the tutorial on coloraxis for more details on coloraxis.

In [6]:

import plotly.express as px
from skimage import data
img = data.camera()
fig = px.imshow(img, color_continuous_scale='gray')
fig.update_layout(coloraxis_showscale=False)
fig.show()

Display multichannel image data with go.Image¶

It is also possible to use the go.Image trace from the low-level graph_objects API in order to display image data. Note that go.Image only accepts multichannel images. For single images, use go.Heatmap.

Note that the go.Image trace is different from the go.layout.Image class, which can be used for adding background images or logos to figures.

In [7]:

import plotly.graph_objects as go
img_rgb = [[[255, 0, 0], [0, 255, 0], [0, 0, 255]],
           [[0, 255, 0], [0, 0, 255], [255, 0, 0]]]
fig = go.Figure(go.Image(z=img_rgb))
fig.show()

Defining the data range covered by the color range with zmin and zmax¶

The data range and color range are mapped together using the parameters zmin and zmax, which correspond respectively to the data values mapped to black [0, 0, 0] and white [255, 255, 255], or to the extreme colors of the colorscale in the case on single-channel data.

For single-channel data, the defaults values of zmin and zmax used by px.imshow and go.Heatmap are the extrema of the data range. For multichannel data, px.imshow and go.Image use slightly different default values for zmin and zmax. For go.Image, the default value is zmin=[0, 0, 0] and zmax=[255, 255, 255], no matter the data type. On the other hand, px.imshow adapts the default zmin and zmax to the data type:

for integer data types, zmin and zmax correspond to the extreme values of the data type, for example 0 and 255 for uint8, 0 and 65535 for uint16, etc.
for float numbers, the maximum value of the data is computed, and zmax is 1 if the max is smaller than 1, 255 if the max is smaller than 255, etc. (with higher thresholds 216 - 1 and 232 -1).

These defaults can be overriden by setting the values of zmin and zmax. For go.Image, zmin and zmax need to be given for all channels, whereas it is also possible to pass a scalar value (used for all channels) to px.imshow.

In [8]:

import plotly.express as px
from skimage import data
img = data.astronaut()
# Increase contrast by clipping the data range between 50 and 200
fig = px.imshow(img, zmin=50, zmax=200)
# We customize the hovertemplate to show both the data and the color values
# See https://plot.ly/python/hover-text-and-formatting/#customize-tooltip-text-with-a-hovertemplate
fig.update_traces(hovertemplate="x: %{x} <br> y: %{y} <br> z: %{z} <br> color: %{color}")
fig.show()

In [9]:

import plotly.express as px
from skimage import data
img = data.astronaut()
# Stretch the contrast of the red channel only, resulting in a more red image
fig = px.imshow(img, zmin=[50, 0, 0], zmax=[200, 255, 255])
fig.show()

Ticks and margins around image data¶

In [10]:

import plotly.express as px
from skimage import data
img = data.astronaut()
fig = px.imshow(img)
fig.update_layout(width=400, height=400, margin=dict(l=10, r=10, b=10, t=10))
fig.update_xaxes(showticklabels=False).update_yaxes(showticklabels=False)
fig.show()

Combining image charts and other traces¶

In [11]:

import plotly.express as px
import plotly.graph_objects as go
from skimage import data
img = data.camera()
fig = px.imshow(img, color_continuous_scale='gray')
fig.add_trace(go.Contour(z=img, showscale=False,
                         contours=dict(start=0, end=70, size=70, coloring='lines'),
                         line_width=2))
fig.add_trace(go.Scatter(x=[230], y=[100], marker=dict(color='red', size=16)))
fig.show()

Displaying an image and the histogram of color values¶

In [12]:

from plotly.subplots import make_subplots
from skimage import data
img = data.chelsea()
fig = make_subplots(1, 2)
# We use go.Image because subplots require traces, whereas px functions return a figure
fig.add_trace(go.Image(z=img), 1, 1)
for channel, color in enumerate(['red', 'green', 'blue']):
    fig.add_trace(go.Histogram(x=img[..., channel].ravel(), opacity=0.5,
                               marker_color=color, name='%s channel' %color), 1, 2)
fig.update_layout(height=400)
fig.show()

Reference¶

See https://plot.ly/python/reference/#image for more information and chart attribute options!